Method, use and device relating to nuclear light water reactors

ABSTRACT

The invention concerns a method of producing and treating a sheet suited to be used as a component or as a part of a component in a fuel assembly for a nuclear light water reactor, which method comprises:
     a) producing a sheet of a Zr-based alloy by forging, hot rolling and cold rolling in a suitable number of steps,   b) carrying out an α+β quenching or a β quenching of the sheet when the sheet has been produced to a thickness which is equal to or almost equal to the final thickness of the finished sheet,   c) heat treating the sheet in the α-phase temperature range of said alloy, wherein the sheet is stretched during the heat treatment according to step c).
 
The invention also concerns a use of a sheet that is produced and treated according to this method, and to methods and fuel assemblies of which said sheet forms a part.

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention concerns a method of producing and treating asheet suited to be used as a component or as a part of a component in afuel assembly for a nuclear light water reactor, which method comprisesthe following steps:

-   a) producing a sheet of a Zr-based alloy by forging, hot rolling and    cold rolling in a suitable number of steps, wherein said alloy    contains at least 96 weight percent Zr and is of such a kind that    the sheet is suitable to be used for said component,-   b) carrying out an α+β quenching or a β quenching of the sheet when    the sheet has been produced to a thickness which is equal to the    final thickness, or at least almost equal to the final thickness, of    the finished sheet,-   c) heat treatment of the sheet in the α-phase temperature range of    said alloy,    wherein step c) is carried out after steps a) and b) have been    carried out.

The invention also concerns a use and devices which will be describedbelow.

The above described method may for example be used for producing channelboxes for fuel assemblies for a boiling water reactor (BWR). Such amethod is for example known through WO-A1-97/40659.

Below first an example of a known fuel assembly for a BWR will bedescribed with reference to FIGS. 1-3.

FIG. 1 thus schematically shows a fuel assembly for a BWR. The fuelassembly comprises a channel box 2 (which here is only shown to theright in the figure). Inside the channel box 2 a number of fuel rods 3are arranged. The fuel rods 3 extend from a top plate 5 to a bottomplate 6. The fuel rods 3 consist of cladding tubes which contain pelletswith nuclear fuel material. In the figure a number of pellets 4 aresymbolically shown. At the top, the fuel rods 3 are provided with endplugs 8. The fuel rods abut against the lower side of the top plate 5with the help of coiled springs 9. A plurality of spacers 7 are arrangedfor holding the fuel rods 3 at a distance from each other. The fuelassembly is long and thus has a longitudinal direction which is hereindicated with a central axis 10. The fuel assembly may often comprise awater channel which usually extends over substantially the whole lengthof the fuel assembly and which enables a flow of non-boiling water upthrough the fuel assembly.

FIG. 2 shows schematically a cross-section of a fuel assembly for a BWR.This cross-section shows that the fuel assembly comprises a centralwater channel 12 with a square cross-section and four smaller waterchannels 14.

FIG. 3 shows schematically a cross-section of another construction of afuel assembly for a BWR. This cross-section shows that the fuel assemblyin this case only comprises one water channel 16 which has a squarecross-section.

Both the above mentioned channel box 2 and the water channels 12, 14 and16 are often produced of sheet materials which are formed and weldedtogether in a suitable manner such as is well known to a person withknowledge within the field. Concerning the channel box 2, this can beproduced by producing two sheets. Each sheet is bent such that aU-shaped profile is achieved. These U-shaped profiles can then be weldedtogether such that a channel box 2 with a square cross-section isobtained. FIG. 4 indicates schematically a cross-section of two suchU-shaped profiles before they have been joined and welded together. FIG.5 shows the channel box 2 when the two U-shaped profiles have beenwelded together. The weld seams are here indicated by 18.

Channel boxes and water channels for fuel assemblies are usuallyproduced in different Zr-based alloys which are well known to a personwith knowledge within the field. For example, the well-known alloysZircaloy-2 and Zircaloy-4 may be used.

In the very particular environment that a nuclear reactor constitutes,the components that form part thereof have to meet many requirements. Avery large number of suggestions for the selection of material and formethods of production of components for fuel assemblies for nuclearreactors have therefore been produced. Even small modifications in thecomposition of alloys or in parameters of production can have a largeimportance for the properties of the components.

The above mentioned WO-A1-97/40659 describes a method of producing sheetmaterial of a Zr-based alloy for producing channel boxes for fuelassemblies for a BWR. According to the described method, a sheet of theZr-based alloy is produced by forging, hot rolling and cold rolling in anumber of steps. Between the rolling steps a heat treatment may becarried out. When the sheet has been produced to the final or almost thefinal dimension it goes through a β quenching. Through the β quenching,the properties of the sheet are improved. Among other things, thecorrosion properties are thereby improved. Furthermore, through the βquenching a more randomised texture of the crystal grains are achieved,which works against the tendency of the sheet to be deformed inparticular selected directions. With a randomised texture is meant thatthe crystal grains are directed randomly in different directions. With anon-randomised texture is thus meant that the crystal grains tend to bedirected in one or some particular directions to a larger extent. Asheet with a non-randomised texture therefore tends to be deformed inparticular selected directions.

In this context it can be noted that the used Zr-alloys exist in anα-phase at lower temperatures (for example at room temperature). In theα-phase, the crystal structure of the material is hcp. At highertemperatures (for example for Zircaloy-4 above about 980° C.) the alloyexists in β-phase. In this phase, the crystal structure is bcc. At atemperature which for example for Zircaloy-4 is between 810° C. and 980°C. the alloy exists in a mixture of α-phase and β-phase, a so-calledα+β-phase.

According to the above mentioned WO-A1-97/40659, the sheet may gothrough a heat treatment in the α-phase temperature range after said βquenching. Thereby the corrosion properties of the sheet are furtherimproved.

SUMMARY OF THE INVENTION

A purpose of the present invention is therefore to provide a method ofproducing and treating sheets which have further improved properties. Apurpose is thereby to increase the flatness and the straightness of theproduced sheet, such that subsequent treatments in order to achieve ahigh flatness or straightness of the sheet can be avoided. A furtherpurpose is to provide such a method which may be carried out withrelatively simple means.

These purposes are achieved with a method comprising the steps of:

-   (a) producing a sheet of a Zr-based alloy by forging, hot rolling    and cold rolling in a suitable number of steps, wherein said alloy    contains at least 96 weight percent Zr and is of such a kind that    the sheet is suitable to be used for said component,-   (b) carrying out an α+β quenching or a β quenching of the sheet when    the sheet has been produced to a thickness which is equal to the    final thickness, or at least almost equal to the final thickness, of    the finished sheet,-   (c) heat treatment of the sheet in the α-phase temperature range of    said alloy,    wherein step c) is carried out after steps a) and b) have been    carried out.

During β quenching, as has been explained above, a phase transformationfrom bcc structure to hcp structure takes place. This phasetransformation partly also takes place at an α+β quenching. Since thephases have different structure and different volume, a change of volumetakes place during this transformation. This change of volume leads tothe fact that large tensions are introduced into the material. Thesetensions lead to the fact that the flatness of the produced sheet maynot be good after such a phase transformation. According to the presentinvention, the sheet is stretched during the heat treatment in theα-phase temperature range of the alloy which is carried out after thequenching of the material. Since the sheet is stretched, the tensionsthat have been created during the above described phase transformationfrom bcc to hcp structure are released. Thereby a flat and straightsheet is obtained. During the heat treatment according to step c) alsoimproved corrosion properties are achieved since this heat treatmentmakes it possible for so-called secondary phase particles to grow. Sincethe sheet is stretched during the heat treatment, the growth ofsecondary phase particles takes place faster since the stretchingincreases the diffusion speed. Since the heat treatment during thedeformation leads to a considerably faster diffusion, it is possible tocontrol the degree of growth of secondary phase particles through theapplied deformation. This is advantageous, in particular for a heattreatment in continuous oven process. For a continuous oven process withconventionally used ovens, it may otherwise be difficult to achieve asufficiently long time of heating in order to obtain the desired growthof secondary phase particles. Since the sheet is stretched during theheat treatment, according to the present invention a sufficient growthof secondary phase particles is achieved also in a continuous ovenprocess.

Preferably, the heat treatment according to above-described step c)should not be carried out during too large of a deformation, since thismay lead to the hcp structure recrystallizing to new and larger grains,which may lead to a randomised grain texture obtained through quenchingthat is deteriorated by recrystallization and grain growth which alsoleads to an impaired ductility because of the grain growth.

According to a preferred manner of carrying out the method, the heattreatment during the stretching is the last heat treatment that thesheet goes through before it is shaped and assembled to the componentfor which it is used. It is however conceivable that a certain heattreatment can be carried out also after the above mentioned step c).Such a heat treatment ought however to be of such a kind that thestructure of the material that is achieved during the heat treatmentduring the stretching is not destroyed.

Possibly, the sheet may be stretched also between the steps b) and c),i.e. also before the heat treatment and the stretching according to stepc). However, normally such a pre-stretching is not necessary.

According to a preferred manner of carrying out the method, step b) is aβ quenching. As has been mentioned above, the quenching may either be anα+β quenching or a β quenching. The best properties of the sheet arehowever obtained by a β quenching. Furthermore, the above mentionedchange of volume which occurs during the stretching is more prominent ata β quenching. The invention is thus particularly advantageously when aβ quenching is carried out in step b).

Preferably, said stretching is carried out at a temperature of at mostthe temperature which constitutes the highest temperature in the α-phasetemperature range of the alloy and at least at the temperature which isabout 70% of the highest temperature with regard to ° K, most preferredat a temperature which is between 80% and 98% of said highesttemperature with regard to ° K. For a continuous oven process, i.e. aprocess where the sheet continuously is moved in an oven, preferably thestretching is carried out at a temperature which is between 90% and 96%of said highest temperature with regard to ° K. For the sake of clarity,it may be noted that for example Zircaloy-4 exists in α-phase up toabout 810° C. and in β-phase over about 980° C. Therebetween, the alloyexists in α+β -phase. Said highest temperature constitutes in this casethus 810° C., which corresponds to 1083° K. For example, a temperatureof 750° C. (which is 1023° K) thus constitutes about 94% of said highesttemperature.

Suitably said stretching is carried out such that the sheet directlyafter having gone through the stretching has a remaining elongationcompared to the state of the sheet immediately before the stretching.The remaining elongation may suitably be at least 0.1%. Even if it ispossible for some alloy that the remaining elongation may exceed 7%, theremaining elongation is, according to a preferred embodiment, between0.1% and 7%, most preferred between 0.2% and 4%. Preferably, thestretching is carried out such that said elongation is lower than thecritical degree of deformation of the alloy. It should be noted thatwhen the sheet cools, it contracts somewhat in accordance with theco-efficient of heat extension of the material. For this reason, theremaining elongation has above been defined for the state in which thesheet is immediately before and after the stretching, i.e. before it hascooled down and thereby contracted because of the temperaturedifference. As is clear from the above mentioned preferred elongations,it is only necessary with a small remaining elongation in order torelease the tensions that have been created during the quenching of thematerial. By the critical degree of deformation is meant the degree ofdeformation where the α-phase grains recrystallize to new and largergrains. Even if the deformation should somewhat exceed the criticaldegree of deformation, only a marginal change of the texture of thematerial is obtained which does not have any larger negative effect onthe material properties. However, the ductility of the material may beeffected negatively if the grains formed are too large. Preferably, thedegree of deformation is therefore lower than the critical degree ofdeformation of the alloy.

The component suitably defines a longitudinal direction which, when thecomponent is used as intended in said fuel assembly, is at leastsubstantially parallel to the longitudinal direction of the fuelassembly. The stretching of the sheet may then suitably be carried outin a direction which corresponds to the longitudinal direction of thecomponent for which the sheet is intended. The sheet may suitably belong and thereby the stretching may suitably be carried out in thelongitudinal direction of the sheet.

Another aspect of the invention concers the use of a sheet produced andtreated according to the method according to any of the above describedmanners. The sheet is thereby used as said component or as part of saidcomponent in a fuel assembly for a nuclear light water reactor. Sincethe produced sheet has good properties and is flat and straight, it issuitably to be used for a component in a nuclear light water reactor.

According to a preferred use, said component is a channel box whichdefines an inner space, inside of the channel box, in the fuel assembly,wherein a plurality of fuel rods are arranged in said inner space andwherein said sheet is used for at least one of the walls of the channelbox.

Preferably, the above mentioned fuel assembly is a fuel assembly for anuclear boiler water reactor.

According to another advantageous use, said component is a water channelarranged in the fuel assembly in order to enable a flow through the fuelassembly of non-boiling water, and wherein said sheet is used as atleast one wall of said water channel.

Since channel boxes and water channels have an extension which normallyconstitutes almost the whole length of the fuel assembly, it isimportant that these components have good properties and good flatnessand straightness. Therefore, with advantage, the sheet produced andtreated according to the method according to the present invention isused for these components.

The invention also concerns a method of producing a channel box for afuel assembly for a nuclear boiling water reactor, which methodcomprises:

producing and treating a plurality of sheets with the method accordingto any of the above described embodiments, and

accomplishing a suitable shape of these sheets and joining the sheetssuch that said channel box is formed.

The invention also concerns a method of producing a water channel for afuel assembly for a nuclear boiling water reactor, which water channelis intended to form part of said fuel assembly for enabling a flowthrough the fuel assembly of non-boiling water, which method comprises

producing and treating a plurality of sheets with the method accordingto any of the above described embodiments, and accomplishing a suitableshape of these sheets and joining the sheets such that said waterchannel is formed.

Through these methods of producing channel boxes and a water channel,respectively, the advantages which are described above concerning theused sheet are obtained.

The invention also concers a fuel assembly for a nuclear boiling waterreactor, comprising:

a channel box with a material structure obtained by the fact that thesheet which forms at least the main part of the walls of the channel boxis produced and treated according to the method according to any of theabove described embodiments, and

a plurality of fuel rods comprising nuclear fuel material arrangedwithin said channel box.

The invention also concerns a fuel assembly for a nuclear boiling waterreactor comprising:

at least one water channel with a material structure obtained by thefact that the sheet that forms at least the main part of the walls ofthe water channel is produced and treated according to the methodaccording to any of the above embodiments.

Also the above described fuel assemblies have advantageous propertiessince the sheet that is used for the walls of the channel box and thewater channel, respectively, is produced and treated with theadvantageous method according to the invention.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a fuel assembly for a nuclear boiling waterreactor.

FIG. 2 shows schematically a cross-section through a fuel assembly for anuclear boiling water reactor.

FIG. 3 shows schematically a cross-section through a boiling waterreactor of another construction.

FIG. 4 shows schematically a cross-section of two U-shaped profilesbefore they are joined.

FIG. 5 shows schematically the U-shaped profiles according to FIG. 4after they have been joined.

FIG. 6 shows schematically a sideview of a sheet in a device with whichthe sheet can be stretched.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

An example of a method according to the invention to produce and treat asheet that is suited to be used as a component or as a part of acomponent in a fuel assembly for a nuclear light water reactor will nowbe described.

As a starting material a Zr-based alloy is used which contains at least96 weight percent Zr. The alloy is of such a kind that it is suited tobe used for for example a channel box or a water channel in a nuclearboiling water reactor. For example, the known alloys Zircaloy-2 orZircaloy-4 may be used. Examples of alloying contents are given in theabove mentioned WO-A1-97/40659. Also other Zr-based alloys which aresuited for the use can be used. It is well known to a person skilled inthe art that the alloying contents which is the case in Zircaloy-2 orZircaloy-4 can be modified in different manners in the order to achievedesired properties. Also for example Zr-based alloys where the largestalloying constituent is Nb may be used. As an example, the use of analloy of Zircaloy-4 will be described below.

An ingot is produced of this alloy. The ingot is forged within theβ-phase temperature range, at a temperature of about 1150° C.

Conventional forging for reducing the thickness of the material iscarried out within the α-phase temperature range. Thereafter thethickness is further reduced by hot rolling after preheating to forexample 950° C. during 15 minutes or 750° C. during 45 minutes. The hotrolling is carried out to a thickness of about 20 mm to 30 mm.Thereafter a second hot rolling follows to a thickness of about 4 mm ata maximum temperature of 650° C.

Between these hot rollings possibly a heat treatment at about 1020° C.during 5 to 10 minutes may be carried out in order to homogenise thealloying elements.

Thereafter a cold rolling is carried out in a number of steps in orderto reduce the thickness to the final thickness or at least almost to thefinal thickness. For example, one to three cold rollings may be carriedout in order to achieve the correct permissible variation of the sheetthickness and the surface finish. Between each cold rolling, thematerial is suitably heat treated at about 730° C. in a continuous ovenprocess.

Thereafter the material is β quenched in that it is heated to about1050° C. during about 10 seconds whereafter a quenching is carried out.The cooling speed may for example be about 25° C. per second.

After the β quenching, the sheet is heat-treated at about 750° C. in acontinuous oven process. The heat treatment may for example take placeduring 2-10 minutes, preferably during about 8 minutes. During this heattreatment, the sheet is stretched such that a remaining elongation ofabout 0.5% is achieved. With remaining elongation is here meant that thesheet has been elongated this much directly after the stretching ascompared to immediately before the stretching. Thereafter possibly thesheet may contract somewhat when it cools in accordance with thecoefficient of heat extension of the material.

FIG. 6 shows schematically how the stretching of the sheet may becarried out in a continuous process. The sheet is fed forward in thedirection that is marked with an arrow with the help of a front pair ofrollers 20 and a rear pair of rollers 22. If the feeding speed with thefront pair of rollers 20 is a little higher than the feeding speed withthe rear pair of roller 22, the sheet goes through a stretching duringthe feed. It should be noted that preferably this feed takes place in anoven such that the sheet is heated at the same time as it is stretched.It should be noted that the stretching does not necessarily have to takeplace in a continuous process with the help of rollers such as is shownin FIG. 6. It is also possible that the sheet is arranged in some otherkind of stretching device in order to carry out the stretching.

Preferably, the sheet is long and the stretching is carried out in thelongitudinal direction of the sheet. This longitudinal direction therebysuitably corresponds to the longitudinal direction of the component forwhich the sheet is to be used.

The produced sheet is preferably used as a component or as a part of acomponent in a fuel assembly for a nuclear light water reactor,preferably a nuclear boiling water reactor. The sheet may for example beused for the channel box 2 which surrounds such a fuel assembly. Anotheruse is for the water channel or the water channels 12, 14, 16 which mayform part of such a fuel assembly.

A channel box 2 for a fuel assembly for a BWR may be produced in thattwo sheets of a suitable dimension are produced. These sheets arethereafter bent to U-shaped profiles such as is shown in FIG. 4.Possibly, the sheets may be heated somewhat, for example to about 200°C., before they are bent. The U-shaped profiles are thereafter weldedtogether in a manner which is well known to a person with knowledgewithin the field. Possibly, the channel box 2 may be shaped in that itis positioned on a fitting of stainless steel, whereafter heating takesplace in order to transfer the shape of the fitting to the channel box2.

A water channel 12, 14, 16 for non-boiling water for a fuel assembly fora BWR may be produced in a similar manner. A sheet is produced with themethod according to the invention. Sheets of suitable dimensions areproduced. These parts are shaped and welded together such that a waterchannel 12, 14, 16 of a suitable shape is obtained. This shape may forexample constitute a cruciform water channel which consists of differentchannel parts 12, 14 such as is shown in FIG. 2 or as a square waterchannel 16 of the kind that is shown in FIG. 3.

The invention also concerns a fuel assembly for a nuclear BWR. Such afuel assembly has a channel box 2 with a material structure, flatnessand straightness obtained in that the sheet which forms the walls of thechannel box 2 is produced and treated according to the method which hasbeen described above. The fuel assembly may for example be of the kindwhich is shown schematically in FIG. 1.

The invention also concerns a fuel assembly for a BWR of which at leastone water channel 12, 14, 16 forms a part. This water channel 12, 14, 16has a material structure, flatness and straightness obtained in that thesheet is produced and treated according to the method which is describedabove. Of course, the fuel assembly may have both a channel box 2 and awater channel 12, 14, 16 which are fabricated of sheets which areproduced and treated with the method according to the invention.

The invention is not limited to the above described examples but may bevaried within the scope of the following claims.

1. A method of producing and treating a sheet suited to be used as acomponent or as a part of a component in a fuel assembly for a nuclearlight water boiling water reactor, which method comprises the followingsteps: a) producing a sheet of a Zr-based alloy by forging, hot rollingand cold rolling in a suitable number of steps, wherein said alloycontains at least about 96 weight percent Zr; b) carrying out one of anα+β quenching and β quenching of the sheet when the sheet has beenproduced to a thickness which is one of equal to the final thickness ofthe finished sheet and approximately equal to the final thickness of thefinished sheet; c) heat treating of the sheet in the α-phase temperaturerange of said alloy, wherein step c) is carried out after steps a) andb) have been carried out, and wherein the sheet is stretched during theheat treatment according to step c); wherein said stretching and saidheat treatment during step c) are carried out in a continuous ovenprocess; wherein said stretching is carried out such that the sheetdirectly after having gone through the stretching has a remainingelongation compared to the state of the sheet immediately before thestretching; wherein said remaining elongation is between about 0.1% andabout 7%; and wherein said component defines a longitudinal directionwhich, when the component is used in said fuel assembly, is at leastsubstantially parallel to a longitudinal direction of the fuel assemblyand wherein said stretching of the sheet is carried out in a directionwhich corresponds to the longitudinal direction of said component forwhich the sheet is intended.
 2. A method according to claim 1, whereinstep b) is a βquenching.
 3. A method according to claim 1, wherein saidstretching is carried out at a temperature of at most the temperaturewhich constitutes the highest temperature in the α-phase temperaturerange of the alloy and at least at the temperature which is about 70% ofsaid highest temperature in ° K.
 4. A method according to claim 3,wherein said stretching is carried out at a temperature which is betweenabout 80% and about 98% of said highest temperature in ° K.
 5. A methodaccording to claim 1, wherein said stretching is carried out such thatsaid elongation is longer than a critical degree of deformation of thealloy.
 6. A method of producing and treating a sheet suited to be usedas a component or as a part of a component in a fuel assembly for anuclear light water boiling water reactor, which method comprises thefollowing steps: a) producing a sheet of a Zr-based alloy by forging,hot rolling and cold rolling in a suitable number of steps, wherein saidalloy contains at least about 96 weight percent Zr; b) carrying out oneof an α+β quenching and a β quenching of the sheet when the sheet hasbeen produced to a thickness which is one of equal to the finalthickness of the finished sheet and approximately equal to the finalthickness of the finished sheet; c) heat treating of the sheet in thea-phase temperature range of said alloy, wherein step c) is carried outafter steps a) and b) have been carried out, and wherein the sheet isstretched during the heat treatment according to step c); wherein saidstretching and said heat treatment during step c) are carried out in acontinuous oven process; wherein said stretching is carried out suchthat the sheet directly after having gone through the stretching has aremaining elongation compared to the state of the sheet immediatelybefore the stretching; and wherein said remaining elongation is betweenabout 0.2% and about 4%; and wherein said component defines alongitudinal direction which, when the component is used in said fuelassembly, is at least substantially parallel to a longitudinal directionof the fuel assembly and wherein said stretching of the sheet is carriedout in a direction which corresponds to the longitudinal direction ofsaid component for which the sheet is intended.